NITROGEN

biochemical transformations of the N cycle - BIOLOGICAL N FIXATION

certain soil microorganisms convert inert atmospheric N₂ gas to N-containing inorganic compounds= plant available

• N2 GAS reduced to NH3

N-fixing microorganisms

some bacteria, actinobacteria, and cyanobacteria)

two groups of N fixing organsims

• freeliving - non symbiotic

• symbiotic - most N fixaton orginates from symbotic fixers

Symbiotic N fixers associated with legumes, nodule-forming

rhizobium bacteria - lives in nodules of legume plants

• this group of microbes fixes the greates amount of N

• bactera recieve photsyntahtes -use as C and Energy source

Symbiotic N fixers associated with non-legumes, nodule-forming

Frankia genus (actinobacteria) and cyanobacteria live in root nodules of alder trees (Alnus sp.) -

• an important species in forests.

Symbiotic N fixers associated with non-legumes, non-nodule-forming

Anabaena is a non-nodule-forming bacteria that live in association with an aquatic fern called Azolla (important in rice production).

Nitrogen mineralization - 3 steps

aminization, ammonification, nitirfication

• mediated by soil organisms and the enzymes they produce

aminization

heterotrophic organisms (breakdown complex soil organic molecules, releasing amines and amino acids.

ammonification

bacteria and fungi convert organic form of N to NH3 and NH4

nitirfication

by certain bacteria - oxidation of NH3 to NO2 to NO3

• plant availabe form that plants grow better in than ammonium

consequence of signifigant nitrififcation

ntirfiation yeilds hydoroen ions = soils can become strongly acidified

nitrogen immoblization

assimaltion of inorganic pant forms of N into microbial biomass - so its no longer avaialbe to plants

denitrfication

reduction of nitrates to volatile N forms (can diffuse out of the soil_

• NO AND N2 gas

consequences of dentifiication

• reduces pool of available N in soil

• adds NO to atmosphere= GHG

nitrate reducton to ammonium

result of anaerobic respriaton of CHEMO-HETERTROPHIC MICROPBES

• similar to dentirification but NO2 is further reduced to NH4+ and bioavialble N isnt lost from the soil

bacteria that carary out nitrification

1st stage (nh4 to NO2) - nitrosomonas

2nd stage (no2 to no3) - nitrobacter

chacterstic of Nitirifying bacteria

• chemo-autotrophs

• function best at neutral PH

nitrate oxidation uses…

the nitrate reductase enzyme, which contains MO

importance of N for plants

• needed for protein synthesis and enzymatic rxns

• compoent of chlorphyll

• compennt of nucleic acid for DNA and RNA (genetic material)

N issues

• not readily avialbe to plants - most is atospheric N

• soil organic compounds contain lots of N but is not avialbe to plants

• all plants need LOTS OF N

SOURES OF SOIL N

• Biological fixation of N2

• Deposition of N compounds (NO3- and NH4+) from the atmosphere by precipitation

• Additions of organic matter (compost, manure)

• Synthetic fertilizer

LOSSES OF SOIL N

• Leaching (NO3-)

• Plant harvesting

• Gaseous losses (volatilization, denitrification)

• Erosion (wind and water)

volitzation of ammonia (NH3)

• in acidic soil - bsorbs ammonia by converting to NH4

• alkaline soil - ammonium forms ammonia

  • higher ammonia conc. = lost to atmoshpehree

why free ammonia in soil solution bad

toxic to microbes and plants in high conc.

ammonium fixaton (NH4) - less avaiialbe N

2:1 phyllosilcate clay minerals - illite and vermiculite

• interlayer region just large enough to hold NH4 ions= ions cant readily escape

• released by weathering

why Nitrate is suspteble to leaching losses

most soils have low anion exchange capaicty, nitrate not strongly absorbed to anion exhcnage sites

problem with nitrogen fertlizers

acidication under certain soil conditions (nitrficaton causing acidic soils)

• Repeated use of large amounts of N fertilizer may require soil amendments to keep the pH from dropping to a sub-optimal level for crop growth.